1. Field of the Invention
The present invention relates to a QPSK (Quadrature Phase Shift Keying) light transmitting section used in an optical communication system of a wavelength division multiplex (WDM) type and more particularly to a QPSK light modulator that compensates the amplitudes of an I component and a Q component of the optical output of a light modulating section forming the light transmitting section so that the amplitudes are equal.
2. Description of the Related Art
For a description of an optical communication system of the wavelength division multiplex (WDM) type using a QPSK light transmitting section, see JP-A-2004-516743.
FIG. 5 is a block diagram of main parts showing one example of the QPSK light transmitting section described in JP-A-2004-516743. In FIG. 5, the modulator of an optical phase transition and modulation device is shown for encoding two 20 Gbit/s NRZ (Non Return to Zero) data streams d1(t) and d2(t) on a single optical carrier wave. This modulator is used, for instance, as part of the transmitter in a WDM optical communication system having modulators for each WDM wavelength channel.
The modulator has a semiconductor laser 1 (laser light source) that needs a stable optical output for a prescribed wavelength that is, for instance, a distributed feedback type (DFB) and the laser also has a constant amplitude, a constant frequency, a constant phase and a single frequency. The laser 1 generates the non-modulated optical output of a selected wavelength (ordinarily, a WDM wavelength channel).
Light [A] from the laser 1 is divided into two parts [B] and [C] by an optical demultiplexer 2. In the illustrated example, light [C] is sent to optical phase modulator 4b via a π/2 phase shift section 3. The optical phase modulators 4a and 4b are respectively formed so that a phase is selectively modulated by 0 or π radian depending on the optical phase modulators in accordance with binary (bipolar) NRZ driving voltages VI(t) and VQ(t).
The optical phase modulators 4a and 4b are respectively formed with, for instance, gallium arsenide or lithium niobate. The optical phase modulators 4a and 4b cannot have a substantial influence on the amplitude (strength) of the optical signal and need to operate as optical phase modulators. To realize this, bias is applied respectively to the optical phase modulators when there is no driving voltage so as to minimize a light transmission. The optical phase modulators are respectively driven when the driving voltages VI (t), VQ (t)=±Vp, and a sudden phase shift is applied to the optical phase modulators while the amplitude modulation is kept minimized. The two phase modulators 4a and 4b have matched delay (phase characteristics) sections.
The optical phase modulators 4a and 4b serve to change the phase of an input light by an electric signal and carry out a binary modulation of phases 0(rad) and π(rad) in the case of a QPSK modulation.
The optical output I[J] of the optical phase modulator 4a and the optical output Q[K] of the optical phase modulator 4b are combined by an optical multiplexer 9 to have a multiplexed and modulated optical output [L].
The modulation patterns (electric signals) of the optical phase modulators 4a and 4b are generated in an IQ encoder 6 and signals are both amplified and amplitude-adjusted by drivers 7a and 7b so as to have a sufficient amplitude for driving the optical phase modulators 4a and 4b. 
FIG. 6 shows a vector notation (constellation diagram) of synthesized modulated optical output [L]. The optical output [J] from the optical phase modulator 4a in FIG. 5 corresponds to I and the optical output [K] from the optical phase modulator 4b corresponds to Q. By respectively changing the I and Q components by 0° and 180°, four patterns of combinations of vectors are formed and the vectors of the IQ synthesized signal are shown by the symbol x.
FIG. 6 shows an ideal constellation since the phases of I and Q components are orthogonal and their amplitudes are equal.
However, a light module such as optical phase modulators 4a and 4b which form a light transmitter or an IQ encoder or a high frequency electric module have large characteristic changes due to temperature or an elapse of time. Accordingly, a problem arises in that a difference is generated between the amplitudes of the optical outputs I and Q.
FIG. 7 shows a state where there is a difference between the amplitudes of optical outputs I and Q such that the amplitude balance between I and Q has collapsed to yield a relation of |I|<|Q|. Such a phenomenon is hypothesized to arise because of IQ transfer differences in the optical demultiplexer, the π/2 phase shifter, the optical phase modulators, and the optical multiplexer, or because of an unevenness in gain characteristics between the driver 7a and driver 7b affects the amplitude difference between I and Q.
As described above, when the constellation deteriorates, it becomes harder for a receiving side (not shown in the drawing) to separate the four symbol positions, and the error rate is worsened and communication quality deteriorates.